X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Wed, 07 Jul 2010 09:32:19 -0400 Message-ID: X-Original-Return-Path: Received: from mta31.charter.net ([216.33.127.82] verified) by logan.com (CommuniGate Pro SMTP 5.3.8) with ESMTP id 4383635 for lml@lancaironline.net; Wed, 07 Jul 2010 09:26:02 -0400 Received-SPF: pass receiver=logan.com; client-ip=216.33.127.82; envelope-from=troneill@charter.net Received: from imp10 ([10.20.200.15]) by mta31.charter.net (InterMail vM.7.09.02.04 201-2219-117-106-20090629) with ESMTP id <20100707132525.HQLN22217.mta31.charter.net@imp10> for ; Wed, 7 Jul 2010 09:25:25 -0400 Received: from [192.168.1.100] ([75.132.241.174]) by imp10 with smtp.charter.net id f1RG1e0073mUFT7051RNSq; Wed, 07 Jul 2010 09:25:23 -0400 X-Authority-Analysis: v=1.0 c=1 a=mI6YO6ZdSLUA:10 a=3oc9M9_CAAAA:8 a=XZ_qCZLNFfFfBz7Ktc4A:9 a=RAcrwjsB9Ni_OjwSZf4A:7 a=DhgM98Z9eknDSTBv9kp2qd3reKMA:4 a=CjuIK1q_8ugA:10 a=U8Ie8EnqySEA:10 a=mEPhNCoaAU41CF2a8lQA:9 a=F0UI24LlMBjBqCXM5zAA:7 a=F3Z8Uamvo3NAEhtUXjwmNgC5kL4A:4 From: Terrence O'Neill Mime-Version: 1.0 (Apple Message framework v1081) Content-Type: multipart/alternative; boundary=Apple-Mail-70--445794993 Subject: Re: [LML] LNC2 CG range Query X-Original-Date: Wed, 7 Jul 2010 08:25:15 -0500 In-Reply-To: X-Original-To: "Lancair Mailing List" References: X-Original-Message-Id: X-Mailer: Apple Mail (2.1081) --Apple-Mail-70--445794993 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=us-ascii Dear humble sevant... or is it 'savant'? See, we often shackle our efforts to understand by having aforehand made = up conventions to better understand something else. I feel motivared to = offer some observations on understanding the L2s, if I may. Don't forget that you have a delete button if this gets too boring. This is how I see the Lancair's wing-flap-stability-stall, as compared = to recent LML discussions of same. The CG range and the wing's characteristics is a good example of = thinking 'in the box'.. Guys working in the wind tunnels measuring = forces of invisible air found that at those small angles where wings = make a lot more z force than Y force -- those forces seemed to be = centered around 25% chord... which they first called a center of = pressure, and then 'aerodynamic center' or a.c., and so they made their = charts about this imaginary point and things work out great for = calculations. The thing is, what's really going on is that as the moving wing flows = through the stationary air, a pressure bubble is generated, with most of = it being at the leading edge, and then tapering 'triangular-sish-ly' = toward the trailing edge.... at least at higher angles of attack. And = the lower or cruise angle so attack the bubble flattens out aft-ward... = and is varied by moving a trailing edge flap. So this is getting at the importance of the bubble... that's what we = move around when we move the pitch control. With a flap, or an aileron, = or a tail flap thing. =20 And so, when we talk about flying with the CG range forward or aft, = we're really talking about the CG moving with respect to this = total-airplane-bubble... which we have to keep centered over our center = of mass. We do this by making little bubbles of pressure on the tail, = or in Lancair reflexing of the flap, by moving the bubble of the wing. = And that's what the CG or center of mass is hanging from. So when we drop a flap, it's obvious that we have moved that big bubble, = and have to balance it with changing the bubble size on the horizontal = tail.,, trim tabbing it. The design characteristics of the L@'s reflexible airfoil are referenced = to the section with the flap not reflexed. Also, therefore, it is most = likely that Lance figured the CG that way, because that's the way the = aerodynamic data was available to him in the NASA report, I think, on = the NLF(1)-0215F... please hasten to correct me if I'm wrong. : ) I just try to keep my (total airplane) bubble's center as close to the = CG, wherever that is, by making a little bubble over or under the = horizontal tail, with the pitch trim tab. One problem is that we pilots don;t talk much abut the shape or location = of the pressure bubble at AOAs higher than the stall angle... and that's = a situation where designers then have to start gluing yukky shapes of = strakes and vanes on to correct this oversight. What Lance should have = done in the first place is wind-tunnel his 200 and -235 from zero AOA up = to 90 degrees, and he would have seen a big forward movement of the = bubble's center resulting from the broad cowl and skinny aft fuselage... = imo. Such testing was done by NASA for the Piper canard (after it crashed) = and on a configuration like the Dragonfly... which I discovered when = researching the Dragonfly we bought... and discussed in a Kitplanes = article many years ago. In the too frequent event that a new design = configuration is locked in by building before testing, the economical = remedy solution then is to fly with a forward CG, or to add strakes aft, = to keep that total-airplane high AOA bubble centered where it belongs = ... aft of the design CG,.. for a restoring pitching moment In the Dragonfly we flew at forward CG. In L235/320 N211AL I have modified the horizontal tail to add slots to = prevent it from stalling at AOA higher than the wing's stall AOA...( = still testing that, but it worked on my Magnum.) Also I added ballast = to the engine mounts to be certain the CG stayed forward, within Lance's = original limits. =20 It's a beautiful little plane, and very efficient ... and this is how it = works -- I think. Terrence L235/320 N211AL On Jul 7, 2010, at 2:18 AM, Sky2high@aol.com wrote: > If you know or even care: > =20 > In general, LNC2's as originally designed seemed to better tolerate a = CG at the forward edge of the envelope rather than flight at or towards = the rear. This includes adequate elevator control at flare during = landing. Lancair tested the long engine mount on an LNC2 that moved the = forward CG edge +1.5" and there were no flight problems. = Hmmmmmm......... > =20 > Consider that the LNC2 wing has a dramatic change in pitch forces when = the flap is moved between its designed standard position and into -7 = degrees reflex. In my airplane at around 140-160 KIAS the difference in = those two flap positions is approximately a measured 6 degrees in = attitude (couldn't measure the AOA delta). It is clear that moving the = flaps sightly out of reflex (1 or 2 degrees) can help resolve = uncomfortable flight at rear CG conditions by pitching the nose down = some and altering the AOA. Perhaps the rear CG and small tail at cruise = leads to some flight instability that cannot be overcome by the size of = the tail?=20 > =20 > So, here is the question: If the CG range was calculated for the = normal state of the wing (flaps not in reflex), is it possible that the = range is too far back for normal cruise flight with the flaps in full = reflex? If so, should the POH aircraft data include two ranges based on = these two flap positions? What does such a change do to the forward CG = limit? > =20 > Of course, this might raise the same question with the 200 series = aircraft. Why? Well, the faired in position of the flaps for 200 = series aircraft is the not-in-reflex position while the plane cruises = with the flaps reflexed and not faired in. The 300 series aircraft has = the flaps in reflex when they are faired in to the fuselage. =20 > =20 > When considering an answer, remember that wings designed to operate by = changing shape (TE goes through some reflex angles) have been primarily = used in tailless airplanes and the TE position controls the pitch = balance of the airplane. I have no idea how the CG range for such an = aircraft is determined. > =20 > Your humble servant, > =20 > Grayhawk > =20 --Apple-Mail-70--445794993 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=us-ascii Dear = humble sevant... or is it 'savant'?

See, we often = shackle our efforts to understand by having aforehand made up = conventions to better understand something else.  I feel motivared = to offer some observations on understanding the L2s, if I = may.
Don't forget that you have a delete button if this gets = too boring.
This is how I see the Lancair's = wing-flap-stability-stall, as compared to recent LML discussions of = same.
The CG range and the wing's characteristics is a good = example of thinking 'in the box'..  Guys working in the wind = tunnels measuring forces of invisible air found that at those small = angles where wings make a lot more z force than Y force -- those forces = seemed to be centered around 25% chord... which they first called a = center of pressure, and then 'aerodynamic center' or a.c., and so they = made their charts about this imaginary point and things work out great = for calculations.
The thing is, what's really going on is that = as the moving wing flows through the stationary air, a pressure bubble = is generated, with most of it being at the leading edge, and then = tapering 'triangular-sish-ly' toward the trailing edge.... at least at = higher angles of attack.  And the lower or cruise angle so attack = the bubble flattens out aft-ward... and is varied by moving a trailing = edge flap.
So this is getting at the importance of the = bubble... that's what we move around when we move the pitch control. =  With a flap, or an aileron, or  a tail flap thing. =  
And so, when we talk about flying with the CG range = forward or aft, we're really talking about the CG moving with respect to = this total-airplane-bubble... which we have to keep centered over our = center of mass.  We do this by making little bubbles of pressure on = the tail, or in Lancair reflexing of the flap, by moving the bubble of = the wing.  And that's what the CG or center of mass is hanging = from.
So when we drop a flap, it's obvious that we have moved = that big bubble, and have to balance it with changing the bubble size on = the horizontal tail.,, trim tabbing it.
The design = characteristics of the L@'s reflexible airfoil are referenced to the = section with the flap not reflexed.  Also, therefore, it is most = likely that Lance figured the CG that way, because that's the way the = aerodynamic data was available to him in the NASA report, I think, on = the NLF(1)-0215F... please hasten to correct me if I'm wrong. : = )
I just try to keep my (total airplane) bubble's center as = close to the CG, wherever that is, by making a little bubble over or = under the horizontal tail, with the pitch trim tab.
One = problem is that we pilots don;t talk much abut the shape or location of = the pressure bubble at AOAs higher than the stall angle... and = that's a situation where designers then have to start gluing yukky = shapes of strakes and vanes on to correct this oversight.  What = Lance should have done in the first place is wind-tunnel his 200 and = -235 from zero AOA up to 90 degrees, and he would have seen a big = forward movement of the bubble's center resulting from the broad cowl = and skinny aft fuselage... imo.
Such testing was done by NASA = for the Piper canard (after it crashed) and on a configuration like the = Dragonfly... which I discovered when researching the Dragonfly we = bought... and discussed in a Kitplanes article many years ago.  In = the too frequent event that a new  design configuration is locked = in by building before testing, the economical remedy solution then is to = fly with a forward CG, or to add strakes aft, to keep that = total-airplane high AOA bubble centered where it belongs ... aft of the = design CG,.. for a restoring pitching moment
In the Dragonfly = we flew at forward CG.
In L235/320 N211AL I have modified the = horizontal tail to add slots to prevent  it from stalling at AOA = higher than the wing's stall AOA...( still testing that, but it worked = on my Magnum.)  Also I added ballast to the engine mounts to be = certain the CG stayed forward, within Lance's original limits. =  
It's a beautiful little plane, and very efficient ... = and this is how it works -- I = think.

Terrence
L235/320 = N211AL



On Jul 7, = 2010, at 2:18 AM, Sky2high@aol.com= wrote:

If you know or even care:
 
In general, LNC2's as originally designed seemed to better tolerate = a CG at=20 the forward edge of the envelope rather than flight at or towards the=20 rear.  This includes adequate elevator control at flare during=20 landing.  Lancair tested the long engine mount on an LNC2 that = moved=20 the forward CG edge +1.5" and there were no flight problems. =20 Hmmmmmm.........
 
Consider that the LNC2 wing has a dramatic change in pitch = forces when=20 the flap is moved between its designed standard position and into -7 = degrees=20 reflex.  In my airplane at around 140-160 KIAS the difference = in those=20 two flap positions is approximately a measured 6 degrees in = attitude=20 (couldn't measure the AOA delta).  It is clear that moving the = flaps=20 sightly out of reflex (1 or 2 degrees) can help resolve uncomfortable = flight at=20 rear CG conditions by pitching the nose down some and altering the = AOA. =20 Perhaps the rear CG and small tail at cruise leads to some flight = instability=20 that cannot be overcome by the size of the tail? 
 
So, here is the question:  If the CG range was calculated for = the=20 normal state of the wing (flaps not in reflex), is it possible that the = range is=20 too far back for normal cruise flight with the flaps in = full reflex? =20 If so, should the POH aircraft data include two ranges based on these=20 two flap positions?  What does such a change do to the forward = CG=20 limit?
 
Of course, this might raise the same question with the 200 series=20= aircraft.  Why?  Well, the faired in position of the flaps for = 200=20 series aircraft is the not-in-reflex position while the plane cruises = with the=20 flaps reflexed and not faired in.  The 300 series aircraft has the = flaps in=20 reflex when they are faired in to the=20 fuselage.     
 
When considering an answer, remember that wings designed to operate = by=20 changing shape (TE goes through some reflex angles) have been primarily = used in=20 tailless airplanes and the TE position controls the pitch balance = of the=20 airplane.  I have no idea how the CG range for such an aircraft is=20= determined.
 
Your humble servant,
 
Grayhawk
 

= --Apple-Mail-70--445794993--